Quantitative analysis may include extent of emphysema, measurement of airway dimensions and extent of air trapping.
The distribution and severity of emphysema may be quantified by CT. CT findings can be assessed by subjective visual methods, density measurements or by post-processing and texture analysis. The simplest method for estimating the severity of emphysema involves assigning a grade based on visual CT interpretation. Typically this approach calls for assessing individual HRCT sections, using 4- or 5-point scale, as either normal or showing mild (<25% of the lung), moderate (25–50% of the lung), marked (50–75% of the lung), or severe (75–100%) of the lung) emphysema, with the total score expressed as percentage of total lung at that level. In general, visual inspection has yielded good correlations between CT and pathological measures of the extent and severity, especially of centrilobular emphysema. However, subjective grading of emphysema appears significantly less accurate than objective CT densitometric results and visual assessment and leads to a significant overestimation of the extent of emphysema. CT densitometry provides better correlation with a morphological reference. The density of emphysematous area is abnormally low. If a histogram plot is made of frequency against pixel density (HU), the emphysematous curve is shifted to the left compared with normal. Voxels with values below a certain number can be highlighted on a CT image (density mask technique) and expressed as a percentage of the total pixels included in the lung section. The use of –960/–970 HU threshold shows optimal anatomical correlation with both macroscopic and microscopic morphometry (the use of –950 HU has proved good correlations as well) and is the optimal threshold to use when evaluating 1-mm section with MDCT acquisition. The percentile density is defined as the value (Hounsfield) at which a certain percentage of the voxels in the frequency distribution histogram have a lower density. Madani et al. showed that the strongest morphometric correlations were observed with using the first percentile on 1.25-mm-thick sectionsobtained after volumetric acquisition using MDCT. It has also become feasible to apply density thresholding technique to volumetric data. Lung volume and volume of emphysema are quantified and displayed at a regional level, for better preoperative assessment in severe emphysematous patients, candidates for bullectomy, lung volume reduction surgery or lung transplantation. Some recommendations have been made in order to standardise sequential CT studies in patients with emphysema: air calibration; image reconstruction with standard reconstruction algorithms; images obtained in deep inspiration (close to total lung capacity) overcome the need for spirometric gating; and not to decrease the dose below 50 mA.
Different methods have also been proposed for quantitative analysis of air trapping extent:
1. The percentage of voxels below –850 HU in expiration.
2. The percentage of voxels between –850 and –910 HU in expiration.
3. The expiration-to-inspiration ratio of mean lung density.
4. The change in relative lung volume with attenuation values from –850 to –950 HU between paired inspiratory and expiratory examinations. This latest method allows the degree of airway dysfunction after extraction of emphysema from the volume of interest.
According to the literature, there is a significant association between the dimensions of the small and large airways in COPD patients, meaning that measuring airway dimension in the larger bronchi can provide an estimate of small airway remodelling.
Specific software permits 3D reconstruction of the lumen of the tracheobronchial tree, followed by an automatic extraction of the central axis of the airways. On any point of the tracheobronchial tree, the software provides a cross-section of the airway at this point strictly perpendicular to the central axis. Then the algorithm permits automatic segmentation of inner and outer contours of the airways, and calculation of wall area (WA), luminal area (LA) and wall area percentage (WA%). Several investigators have used this technique in COPD patients.
Bronchial wall thickening in cigarette smokers has been related to lung function (FEV1). In COPD patients, bronchial wall thickening and extent of emphysema are both the strongest determinants of FEV1. Both bronchial wall thickening and the percentage of emphysema extent have been demonstrated to be associated with COPD exacerbation frequency.
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